1. Field of the Invention
This invention relates to a dental article such as an orthodontic bracket, orthodontic arch wire, or crown substrate having one or more surfaces covered with a hard, relatively thin carbon coating.
2. Description of the Related Art
Metal alloys are widely used to make articles for dental treatment. For example, stainless steel is commonly used to make tooth brackets, buccal tubes, arch wires and bands for orthodontic treatment. Other alloys are used for substrates that serve as bases or copings for porcelain in making a crown.
Often, metal alloys used for dental articles are exposed in the oral cavity and come into contact with oral mucosa and saliva for extended periods of time. Orthodontic brackets, for example, are mounted over external tooth surfaces and are generally in continuous contact with saliva during treatment. Such brackets may also intermittently contact the patient's tongue or inner portions of the patient's cheeks or lips. Orthodontic buccal tubes, arch wires and bands are similarly exposed. The metal substrate of dental crowns and bridges is substantially covered by porcelain but is exposed at the margin next to the gingiva and as a result is in contact with saliva and sometimes the gingiva or other oral tissues.
It has been widely reported that certain metals cause an allergic reaction in a certain percentage of the population. These metals are commonly used in dental articles, and it is suspected that the metals diffuse from such dental articles into the patient and cause an allergic reaction in some patients. For instance, reports in the literature indicate that nickel is suspected of causing inflammation (and sometimes rashes or blisters) of oral tissue. Rapid loss of alveolar bone has been reported to be associated with the use of metal alloy crowns in patients with nickel hypersensitivity.
The concentration of nickel in dental articles often varies from manufacturer to manufacturer, and may also vary for different articles. Orthodontic appliances such as brackets, buccal tubes, bands and arch wires are commonly made of stainless steel having a nickel concentration ranging from 8 to 17 percent by weight. Metal substrates for dental crowns and bridges are often cast of an alloy having a nickel concentration of about 70 percent by weight. Some arch wires are made of a nickel-titanium alloy having a nickel content of 50 to 55 percent by weight.
Chromium in metal alloys of dental articles is also suspected to cause an allergic reaction in some patients. The chromium concentration in orthodontic stainless steel appliances often ranges from 17 to 20 percent by weight. The chromium concentration of alloys for crown and bridge substrates often ranges from 12 to 30 percent by weight.
Corrosion has also been reported as a significant problem in conventional dental articles made of metal alloys. Corrosion due to salts and acids in the oral cavity may corrode and pit the surface of metal dental articles, establishing locations where bacteria may accumulate. The accumulation of bacteria is particularly a problem in areas that cannot be reached by a toothbrush.
Dental articles are sometimes made of materials other than metal alloys containing nickel and chromium and can be used for patients sensitive to these elements. For instance, orthodontic brackets may be made of monocrystalline or polycrystalline alumina. However, stainless steel brackets are generally lower in cost than ceramic brackets and are preferred by many orthodontists. Ceramic brackets are also known to present higher frictional resistance to arch wires, a disadvantage in that the increased friction slows movement of the teeth and can lengthen treatment time.
Coating dental articles with a synthetic resinous material has previously been suggested. For example, U.S. Pat. No. 4,050,156 describes articles coated with a layer of para-oxybenzoyl homopolyester and polytetrafluoroethylene and a pigment for providing tooth coloring. Other aesthetic coatings made of polymeric materials are described in U.S. Pat. Nos. 3,504,438, 4,722,689 and 4,946,347.
Additionally, U.S. Pat. No. 4,626,209 describes an orthodontic bracket having a bonding base coated with a corrosion resistant metallic powder selected from the group consisting of stainless steel, nickel alloys, cobalt alloys, titanium and titanium carbide in order to enhance the bond of the base to the tooth. U.S. Pat. No. 4,902,224 describes an orthodontic bracket base that is coated with a siliceous material to enhance the adhesive bond to orthodontic cement. A tooth articulator described in U.S. Pat. No. 4,780,079 has a cobalt coating to kill bacteria.
While the patents identified above relate to coated dental articles, the problem of nickel or chromium diffusion from the alloy is not mentioned. There remains a need in the art for a coated dental article that is made of materials that are relatively inexpensive and yet reduces the likelihood of allergic reaction reportedly caused by conventional dental articles.